Numerical Analysis for Voltage Responsivities of

an Improved Multi-Frequency Band Pyroelectric

Sensor

Chun-Ching Hsiao and Sheng-Yi Liu Department of Mechanical Design Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei Township,

Yunlin County 632, Taiwan

Email: cchsiao@nfu.edu.tw, leouishen@gmail.com

Abstract—This article proposes an improved multi-

frequency band pyroelectric sensor for extending the

sensing frequency with a fine voltage responsivity to detect

subjects with various velocities. The proposed sensor is built

on a silicon substrate, consisting of four pyroelectric layers

with various thicknesses deposited by sputtering and aerosol

deposition (AD) method, top and bottom electrodes and a

silicon nitride layer. The thinnest ZnO layer deposited by

sputtering with a small thermal capacity and a rapid

response shoulders a high-frequency sensing task, while the

thicker ZnO layers deposited by the AD with a large

thermal capacity and a tardy response shoulders a low-

frequency sensing task. The improved design is further for

redeeming drawbacks of thicker pyroelectric layers with a

tardy response at a high-frequency band and a thinner

pyroelectric layer with a low voltage responsivity at a low-

frequency band. The improved multi-frequency band

pyroelectric sensor is successfully designed and analyzed in

the present study, which could indeed extend the range of

the multi-frequency band sensing.

Index Terms—pyroelectric sensor, multi-frequency, zinc

oxide, thin film, aerosol deposition, sputtering

I. INTRODUCTION

Pyroelectric sensors function by making use of the

pyroelectric effect. The pyroelectric effect has been

applied to environmental energy-harvesting systems.

Pyroelectric energy conversion also offers a novel and

direct way to convert time-dependent temperature

fluctuations into electricity for micropower generators

and low-energy-consumption systems [1]-[3].

Pyroelectric devices are useful in many applications, such

as pollution monitoring, hot image detectors, intruder

alarms, gas analysis and temperature sensors. Thin-film

pyroelectric sensors have many advantages, such as

integration with on-chip circuitry, uncooled detection,

room-temperature operation, speed, lower system costs,

portability and a wide spectral response with high

sensitivity [4]-[7]. A general pyroelectric sensor consists

of a pyroelectric layer sandwiched between top and

bottom electrodes, which is built on thermal-isolation

structures or substrates for minimizing heat loss. The

Manuscript received December 15, 2014; revised April 24, 2015.

dynamic pyroelectric response current (ip) of pyroelectric

sensors is proportional to the absorption coefficient of

radiation (η), the pyroelectric coefficient of the

pyroelectric film (P), the electrode area (A) and the

temperature variation rate of pyroelectric films (dT/dt).

At a low frequency (ω<<τT-1

), Rv is proportional to the

frequency, and is shown as the following equation:

𝑅𝑣 =𝑅𝐺𝜂𝑃𝐴𝜔

𝐺𝑇 (1)

Equation (1) can easily maximize Rv by minimizing GT

(i.e., by adding a thermal insulation layer between the

pyroelectric film and the substrates, adopting a suspended

structure fabricated by a bulk micromachining technique

using anisotropic silicon etching or by using a substrate

with a low thermal conductivity). At a high frequency

(ω>>τT-1

; ω>>τE-1

), Rv is inversely proportional to the

frequency, and is shown as the following equation:

𝑅𝑣 =𝜂𝑃

𝑐′𝑑(𝐶𝐸+𝐶𝐴)𝜔 (2)

Equation (2) can easily maximize Rv by minimizing the

thermal capacity of the pyroelectric element H (i.e.,

decreasing the thickness of the pyroelectric element).

Hence, the frequency at τT-1

is a watershed to distinguish

the ranges of low and high frequencies, and the

pyroelectric element’s thickness determines the value of

the thermal time constant (τT = c’×d×A/GT) under the

decided pyroelectric materials and electrode areas.

Therefore, a thicker pyroelectric element increases the

thermal time constant, which is suitable as the sensor for

a low-frequency range. Unlike the thicker element, a

thinner pyroelectric element reduces the thermal time

constant, which is suitable as the sensor for a high-

frequency range. Therefore, in the present study, an

improved structure consisted of four pyroelectric ZnO

films with various thicknesses, top and bottom electrodes,

a thermal isolation layer and a substrate for designing an

improved multi-frequency band pyroelectric sensor. The

four pyroelectric layers were mainly deposited by

sputtering and aerosol deposition (AD). The thinnest

layer in pyroelectric layers was grown by sputter, the

others were deposited by the AD with shadow mask

method. The improved design was further for redeeming

drawbacks of a thicker pyroelectric layer with a tardy

International Journal of Electronics and Electrical Engineering Vol. 4, No. 1, February 2016

©2016 Int. J. Electron. Electr. Eng. 39doi: 10.18178/ijeee.4.1.39-42

response at a high-frequency band and a thinner

pyroelectric layer with a low voltage responsivity at a

low-frequency band.

Zinc oxide (ZnO) is an important material in the

electronic industry. It is a II-VI oxide semiconductor. It

has also high ionicity compared with Si,Ge and III-V

compounds. Besides, ZnO has some distinct features like

non-stoichiometric defect structure, anisotropy in crystal

structure, large direct band gap, strong absorption in the

UV region, transparency in the visible region, large

variation of conductivity, and high surface sensitive

catalytic activity in different ambiances. These properties

make it useful in many applications, such as pyroelectric

devices [6], [7], gas sensors [8] and film bulk acoustic

resonators [9]. The thinnest ZnO film was deposited by

RF sputtering. Sputter deposited films have a

composition close to that of the source material.

Moreover, the thicker ZnO film was grown by the AD.

The AD provides many advantages for producing films in

the range of 1~100μm thickness with a high deposition

rate, low deposition temperature and low cost. The AD

method can achieve fine patterning and fabricate a dense

structure by the reduction of crystallite size by fracture or

plastic deformation at room temperature [10]-[12]. In the

present study, transient temperature fields in improved

multi-frequency band pyroelectric sensors were simulated

to probe into temperature variation rates at various

pyroelectric layers and estimate the voltage responsivity

of the sensors.

II. MATERIALS AND METHODS

The improved multi-frequency band pyroelectric

sensor, consisting of four ZnO pyroelectric layers with

various thicknesses, and top and bottom electrodes, was

built on a silicon substrate with a thermal-insulation

(silicon nitride) layer to reduce heat and electric loss. Fig.

1 shows the schematic diagram of the improved multi-

frequency band pyroelectric sensor. The thinnest ZnO

pyroelectric layer was deposited by sputtering, and the

others were deposited by the AD. The sputtered ZnO

layer acted as a producer of the responsivity at higher

frequency bands, while the aerosol ZnO layers detected

the signals of the sensors at lower frequency bands.

Figure 1. Schematic diagram of the improved multi-frequency band

pyroelectric sensor (unit: μm).

The improved multi-frequency band pyroelectric

sensor used both sputtering and AD to extract the

advantages of the thin and the thick ZnO pyroelectric film

to further integrate the electrical outputs. Temperature

variation rates in pyroelectric layers markedly affect the

responsivity of pyroelectric sensors. The response is in

proportion to the temperature variation rate in

pyroelectric layers. The dynamic pyroelectric response

current of pyroelectric devices can be expressed as [4]:

ip = η×P×A×dT/dt (3)

That is, higher the temperature variation rate in

pyroelectric films, higher response of pyroelectric sensors.

However, the temperature variation field is difficult to

extract from thin films by experimental measurements. In

the present study, a two-dimensional finite element model

was constructed using the commercial multiphysics

software COMSOL MULTIPHYSICS® 4.2 to explore

the temperature variation rate in the multi-frequency band

ZnO pyroelectric sensors. The material properties of the

films and substrate are shown in Table I. There was an

isotropic assumption for the films and substrate properties

in this model. The model was meshed using a regular

mesh, as shown in Fig. 2. The thickness of the sputtered

ZnO layer (TPZ) was fixed as 0.3μm, while the

thicknesses of the aerosol ZnO layers (TAZ1, TAZ2, TAZ3)

were 3, 1 and 0.6μm. The incident irradiation power

applied on the top side of the multi-frequency band

pyroelectric device was nearly 1.228×10-12

W/μm2 [13].

The thermal isolation condition was applied to the rear

side of the silicon substrate, and the symmetric condition

was applied to the two lateral sides as boundary

conditions.

In fact, pyroelectric devices use the voltage

responsivity for presenting the performance of the sensors.

Hence, the voltage responsivities were calculated for

estimating the electrical outputs of the sensors. The

voltage responsivity (Rv) is defined as the signal

generated when pyroelectric sensors are exposed to a

modulated radiation. Moreover, when pyroelectric

sensors are connected to a high impedance amplifier, the

observed signal is equal to the voltage produced by the

charge. Rv can be expressed as [4]:

𝑅𝑣 =𝑖𝑝

𝑌×𝑊0 (4)

where ip is the dynamic pyroelectric response current, W0

is the magnitude of the incident radiation and Y is the

electrical admittance, as:

𝑌 = 𝑅𝐺−1 + i𝜔𝐶𝑇 (5)

where RG is the gate resistor, ω is the modulated

frequency of the incident radiation, CT is the sum of CE

and CA, CE is equal to ε0×εr×A/d, ε0 is the vacuum

permittivity (8.85×10-12

F/m) and εr is the relative

permittivity or dielectric constant of the materials. Rv was

estimated using the simulated data of the temperature

variation rates and (3), (4) and (5). The relative

conditions for computing the voltage responsivity are

shown in Table II.

International Journal of Electronics and Electrical Engineering Vol. 4, No. 1, February 2016

©2016 Int. J. Electron. Electr. Eng. 40

TABLE I. MATERIAL PROPERTIES USED FOR THE SIMULATION OF

TEMPERATURE VARIATION FIELDS

Materials

Thermal

conductivity

(Wm−1·K−1)

Specific

heat

(Jg−1·K−1)

Density (g·cm−3)

Thick-

ness

(μm)

Silicon substrate

163 0.703 2.330 5

Silicon nitride 20 0.700 3.100 1

Electrodes 317 0.129 19.300 0.1

Sputtered and aerosol

ZnO layers

6 0.125 5.676

TPZ,

TAZ1, TAZ2

and

TAZ3

TABLE II. RELATIVE CONDITIONS FOR ESTIMATING RV

RG

(MΩ)

CA

(pF)

P

(10-4

C/m2·K)

A

(mm2)

εr

(Unit-less)

W0

(W/μm2)

22 6 0.1 9 11 1.228 × 10-12

Figure 2. Two-Dimensional numerical model for the improved multi-

frequency band ZnO pyroelectric sensor.

Figure 3. Schematic diagram for electrical signal treatment procedure.

The improved multi-frequency band pyroelectric

sensor redeemed the drawbacks of the thinner and the

thicker ZnO pyroelectric film to integrate the electrical

outputs generated from those films into an all-round

signal. A schematic diagram of the signal treatment is

shown in Fig. 3. The voltage responsivity of VP was

generated from the sputtered ZnO film for shouldering a

high-frequency response, and the voltage responsivities

of VA1, VA2 and VA3 were generated from the aerosol

ZnO films for taking a low-frequency response. The

integrated and treated electrical signal was VT. The

responsivities of VP, VA1, VA2 and VA3 were filtered,

amplified, modulated and combined as an integrated

voltage responsivity (VT) by NI LabVIEW software.

Moreover, both the integrated voltage responsivity of the

sensors and the reference signal of the photodiode were

acquired, recorded and displayed using NI LabVIEW

system consisted of a case of NI PXIe-1082, a controller

of NI PXIe-8135, a data acquisition card of NI PXIe-

6366 and NI LabVIEW 2013 software.

(a)

(b)

(c)

Figure 4. Voltage responsivities of the improved multi-frequency band

pyroelectric device with TPZ=0.3μm, TAZ1=3μm, TAZ2=1μm and

TAZ3=0.6μm under the incident irradiation power modulated with

various chopping frequencies of about (a) 14KHz, (b) 33KHz and (c) 100KHz.

International Journal of Electronics and Electrical Engineering Vol. 4, No. 1, February 2016

©2016 Int. J. Electron. Electr. Eng. 41

III. RESULTS AND DISCUSSION

It clearly shows that a large and rapid temperature

variation is a favorable condition for generating high

electrical responsivity. Therefore, transient temperature

fields in the multilayer ZnO thin film pyroelectric devices

were simulated, and the response time of the devices was

estimated. The data about temperature variation rates and

response time were further used to calculate the voltage

responsivity of pyroelectric sensors.

The transient temperature fields with various chopping

frequencies were simulated for further calculating voltage

responsivities when the improved multi-frequency band

pyroelectric devices were exposed to irradiation power

with various chopping periods. A square waveform was

produced using a programmable function generator to

modulate the irradiation power with chopping frequencies

of about 14K, 33K and 100K Hz. Fig. 4 shows the

voltage responsivities of the improved multi-frequency

band pyroelectric device with TPZ=0.3μm, TAZ1=3μm,

TAZ2=1μm and TAZ3=0.6μm when the incident irradiation

power was modulated with various chopping frequencies

of about 14K, 33K and 100KHz. The shape of the

integrated voltage responsivity of VT at three chopping

frequencies almost approached to a square wave.

Furthermore, the amplitude of the integrated voltage

responsivity of VT at three chopping frequencies was

nearly identical. Therefore, VT generated by gradually

increased thickness of the ZnO layers had a

compensatory effect among the ZnO layers with various

thicknesses. This implied that the integrated voltage

responsivity of VT generated by the improved multi-

frequency band pyroelectric devices could present the

performances of various pyroelectric devices at various

sensing frequency bands.

IV. CONCLUSION

The general pyroelectric sensor with a single

pyroelectric layer has only a single sensing frequency

band. However, multi-frequency sensing tasks could

improve applications of pyroelectric sensors for detecting

subjects with various speeds or frequencies under a fine

responsivity. A improved multi-frequency band

pyroelectric sensor was successfully designed and

analyzed in the present study. The combination of a

thinner sputtered ZnO layer and three thicker aerosol

ZnO layers proved useful in the present design. The tactic

by gradually increased thickness of the ZnO layers

presented a compensatory effect for redeeming the

drawbacks among the ZnO layers. The ranges of the

multi-frequency sensing tasks could be predicted by the

proposed design and analysis.

ACKNOWLEDGMENT

The authors are grateful for the financial support from

the National Science Council of Taiwan through Grant

Nos. NSC 102-2221-E-150-037 and MOST 103-2221-E-

150-038, and the experimental support from the Micro-

Device Laboratory at the National Formosa University.

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Chun-Ching Hsiao is an associate professor of

the Department of Mechanical Design

Engineering at the National Formosa University, Taiwan. His researches focus on pyroelectric

sensors, flexible devices, thermoelectric

generators, pyroelectric harvesters, pyroelectric

human-machine interface and residual stress of

thin film.

Sheng-Yi Liu is a graduate student of the

Department of Mechanical Design Engineering

at the National Formosa University, Taiwan. His research interests are computer simulation,

fabrication on ZnO pyroelectric sensors and

micro-manufacture.

International Journal of Electronics and Electrical Engineering Vol. 4, No. 1, February 2016

©2016 Int. J. Electron. Electr. Eng. 42